Tom Steyer, in a crisp blue oxford and a Royal Stewart tie, is sitting in a wood-paneled restaurant in Arlington, Virginia, now with two fresh-faced media handlers by his side, both in jackets and ties and feverishly punching out emails on BlackBerries. It's November 2013, the day after the Virginia race for governor, and Steyer is philosophical, mulling over what his money has bought him here. Democrat Terry McAuliffe has eked out a two-percentage-point win over Republican Ken Cuccinelli, a climate-change denier.

It wasn't clear whether Steyer's cash infusion in the race – $8 million all told – had been a crucial factor. But for him, winning the race was only half the point. He wanted to turn McAuliffe's opponent into a virtual billboard for public awareness, and he convinced himself that he'd achieved that goal. "Not acknowledging that there's an energy climate problem was one of the reasons that Cuccinelli was disqualified from being a governor of Virginia," he says, "because he's too out of touch. If Americans decide that if you won't accept basic science, you're too out of touch to hold a basic position – that's good. That's really important."

This was Steyer's second big intervention in a political race with his super PAC. His first strike was in Massachusetts last spring during the Senate race to replace John Kerry, then incoming secretary of state. Steyer made an immediate splash by flying a banner over Boston that read steve lynch for evil oil pipeline. Edward Markey won the race handily, and Steyer managed to successfully introduce himself as the new rich guy ready to rumble on climate.

"We made them talk about climate," he says. "We made the people of Massachusetts talk to each other and think about climate and think about what's responsible to do, and that's a huge change."

The Massachusetts and Virginia races were only a beta test for the upcoming midterm elections, says Steyer's adviser Chris Lehane – Steyer will inject millions into assorted races in a much more dangerous election cycle for the Democrats, when Republicans are threatening a wholesale takeover of Congress. But all the while, Steyer has stayed focused on the White House. After we met in San Francisco last spring, he threw a lavish fundraiser, where he personally urged President Obama to stop the pipeline. Obama listened respectfully but explained that halting it was difficult without convincing people that its economic benefits were overstated. Steyer saw this as an opening, explaining to Obama that talk of the pipeline creating 6,500 American jobs was a fantasy cooked up by the oil companies. Steyer says it will create only 35 jobs. "We were saying all along that these job numbers are fictitious," he recounts. "Here's what they actually believe, what they put down in black-and-white. Just take their numbers. And the president took those two points. We were talking about the economics of this."

Obama had too much political pressure from the right on health care and the budget to fight for climate just yet. But he was getting enough push from his political left, not least from Steyer, that he declared last summer that he was going to evaluate the Keystone project "based on whether or not this is going to significantly contribute to carbon in our atmosphere," adding that there was "no evidence" that the pipeline was a significant generator of jobs. Though many observers read Obama's statement as ambiguous, a stopgap to give him time to stave off the issue so he could fight other battles in the short term, Steyer saw a marked shift – and his own fingerprints.

"We talked about the impact of gasoline prices in the Midwest – which he quoted!" he hooted. "To me, it looked fairly straightforward, like he was giving himself space to say no. And that doesn't mean he's definitely going to say no. He was refuting the arguments as to why this was an important thing to say yes to. From my point of view? For him? This is an obvious decision."

With the politics getting increasingly hazardous for Obama, Steyer says the president will come to his senses on the pipeline because it's an easy way to claim a legacy by fiat, without a battle in Congress: He simply cancels the license for the Keystone XL pipeline and he's an automatic hero for history. In that sense, Obama and Steyer are in this one together. "Everyone thinks that Obamacare is going to be his legacy, and it will be in the short run," Steyer says. "But his legacy issue 20 years from now will be this. And a hundred years."

One positive sign did emerge after the Virginia race: In December, Steyer's friend John Podesta was hired by Obama as a counselor, putting an important Steyer ally and climate advocate within arm's length of the president and the Keystone decision.

Another positive: Steyer seems to be having fun. "It's unpleasant to target people with bad messages and make them miserable," says Taylor. "A lot of people wouldn't want to do that – personally, by professionals, with repercussions. But he's quite willing. He gets energy from combat."

Meanwhile, Steyer is teaming up with his old Goldman Sachs pals, Hank Paulson and Robert Rubin, along with Mayor Michael Bloomberg, to finance a study on the costs of inaction in climate change. Once the data is out there, Steyer figures, no candidate who wants to win political office will be able to ignore it. "You can't be president of the United States if you don't believe in gravity," he says. "It's just dumb. Too much stuff depends on us accepting data and using it to inform our decisions."

Logic and data and money are indeed powerful instruments. They were certainly the lifeblood of Tom Steyer's career. But the political path to success on climate-change activism, from the trenches of state races to the campaign to influence a president, is an incremental slog. By comparison, the golden ladder of an all-American overachiever, from Exeter to Yale to Stanford to Goldman and beyond, looks well-defined. And indeed, Steyer is not discounting a return to that ladder – in particular, the well-trod route of wealthy donors to positions of power in the White House. Already, his closest associates are speculating about what high office Steyer might be suitable for if, for instance, he is tapped by a future president Hillary Clinton in 2016. Treasury secretary? Secretary of energy?

Says his brother Hume, "If she does win, she might give him a job. And if she offered him a job, he might take it."

Barring that, he could run for office in California, where he has promised to pour more millions into a new campaign to tax all oil extracted in the Golden State. As he climbs into a minivan to be driven to the airport, where he'll fly commercial home to San Francisco from Washington, Steyer keeps the door wide open. "I'm not doing this to build my résumé," he says. "I'm doing something to get something done."

Then he flashes a smile that is worth a billion dollars. "If I could do something that would be helpful," he says, "I would do it."